The present invention generally relates to image data compression systems and facsimile machines employing the image data compression system, and more particularly to an image data compression system which compresses a quantity of data to be transmitted when transmitting image data such as characters and patterns and a facsimile machine employing the image data compression system.
A modified READ (MR) coding scheme is used as a standard two-dimensional data compression system of facsimile machines. A description will be given of a case where a character "A" having a pattern shown in FIG. 1 is coded by the MR coding scheme.
For the sake of convenience, FIG. 1 shows a screen having 16 picture elements along a horizontal axis (X-axis) and 11 picture elements along a vertical axis (Y-axis). It is assumed that picture elements constituting a column on the right of a rightmost column within the screen are all black picture elements as indicated by a hatching.
The pattern of the character "A" is developed on the screen as a graphic, and MR codes shown in the following Table 1 are obtained by scanning this screen in the direction of the X-axis.
TABLE 1 ______________________________________ Line No. Code Sequence No. of Bits ______________________________________ 1 V(0) 1 2 H{W(7)B(2)}V(0) 10 3 V(0)V(0)V(0) 3 4 VL(1)VR(1)V(0) 7 5 VL(1)VR(1)V(0) 7 6 VL(1)H{B(3)W(2)}VR(1)V(0) 16 7 VL(1)VL(1)VR(1)VR(1)V(0) 13 8 VL(1)PVR(1)V(0) 11 9 VL(1)H{B(4)W(6)}VR(1)V(0) 17 10 V(0)VL(1)VR(1)V(0)V(0) 9 11 PP 8 ______________________________________
The MR code is basically a vertical code. The MR coding scheme codes a shift quantity of an X-axis address of a transition point where a picture element undergoes a transition from white to black or black to white on a present scanning line which is being processed when compared with an X-axis address of a transition point where a picture element undergoes a transition from white to black or black to white on a previous scanning line which is immediately prior to the present scanning line.
In the Table 1, a value m in brackets "( )" indicates the shift quantity, where m.gtoreq.3. Hence, V(0) indicates that there is no shift. On the other hand, VR(m) and VL(m) respectively indicate shifts to the right and left. In addition, H indicates that a modified Huffman (MH) coding scheme is applied. The MH coding scheme is used when the shift quantity is greater than "3" and a new pair of transition points where the picture element undergoes a transition from white to black or black to white is generated. W(p) and B(p) of the MH code respectively indicate that p consecutive picture elements are white and p consecutive picture elements are black. P indicates that two codes forming a pair are deleted.
Each number under the column "No. of Bits" in the Table 1 indicates a total number of bits of the corresponding code sequence under the column "Code Sequence". The following Table 2 shows the correspondence of the codes and the number of bits.
TABLE 2 ______________________________________ Code Name Code No. of Bits ______________________________________ Pass P 0001 4 MH Code H 001+MH(p1)+MH(p2) 3+(H1+H2) MR Code V(0) 1 1 VR(1) 011 3 VR(2) 000011 6 VR(3) 0000011 7 VL(1) 010 3 VL(2) 000010 6 VL(3) 0000010 7 ______________________________________
In the Table 2, second and third terms under the column "Code" of the MH code respectively are MR codes indicating the numbers of consecutive white picture elements and consecutive black picture elements.
In a general graphic, the shift quantity of the X-axis address of the transition point of the picture element is in many cases constant and continuous. For example, in the case of the character "A" shown in FIG. 1, the shift quantity is 0 or .+-.1 and the same shift quantity such as .+-.1 continues between many scanning lines.
However, as described above, the MR code is basically a vertical code. The MR coding scheme codes a shift quantity of an X-axis address of a transition point (or edge) where a picture element undergoes a transition from white to black or black to white on an object scanning line when compared with an X-axis address of a transition point (or edge) where a picture element undergoes a transition from white to black or black to white on a reference scanning line. For this reason, when transmitting the image data, it is necessary to transmit two kinds of data respectively indicating a value of the shift quantity (that is, a difference between the X-axis addresses) and a direction of the shift. Therefore, there is a problem in that there is a limit to further reducing a quantity of data which needs to be transmitted when making an image data transmission.